The acquisition of ultrasonic image profiles, also referred to as acoustic images, is widely used for geological analysis of oil reservoirs. These images allow for textural, structural, geometric and reservoir quality analysis by means of the acoustic properties of the rocks. This analysis allows for characterization of reservoirs by the types of rock present and by productivity potential.
Ultrasonic image profiles are acquired using a tool capable of emitting acoustic wave pulses that propagate through the drilling fluid until they reach the wall of the well, where they are reflected and bounce back to the tool that registers transit time and amplitude properties through the transducer.
These profiles are acquired in uncoated boreholes, such as oil wells. The acoustic pulses are emitted along the entire circumference of the wall of the wells. After they are reflected, these pulses are measured around the entire acquisition tool.
Sound waves are reflected due to the contrasting impedance between the fluid-rock medium. The higher the contrast in impedance, the greater the amplitude of the reflected waves. The graphical representation of the amplitude and transit time measurements at depth generates a 360° image of the profiled section of the well.
The emission of acoustic waves usually takes place through a transducer, which is a piezoelectric crystal, namely a crystal capable of transforming an electrical pulse into a pulse of mechanical waves and vice versa.
FIG. 1 illustrates a cross section of a well from which an image profile is to be generated, wherein tool 1 is positioned as described above. Tool 1 is represented in the center of the well, which has a transducer 11 for emission of ultrasonic waves. The waves propagate through drilling fluid 2 until they reach wall 4 of the well, at which point they are then reflected off wall 4 of the reservoir, returning to transducer 11, which also has the means of recording the reception of such waves.
As described above, based on the transit time and amplitude records of the waves, it is possible to determine a number of characteristics of rock 3 of the profiled section of the reservoir, generating 360° images (a complete vision of a section) of the well wall.
However, the tool used to generate this image is subjected to the eccentricity of its position, causing attenuation of the acoustic waves to be related to the geometrical effect and not to the acoustic properties of the reservoir as is desired.
The eccentricity of the position of the tool can be caused by both inefficiencies in centralizing the tool during the acquisition of the profile and by variations in well geometry. Oil wells are ideally produced with cylindrical geometry, however, it is extremely common for there to be variations in this geometry, mainly due to oscillations of the drill bits whilst the well is being drilled. FIG. 2a (taken from the book “novel mechanical caliper image while drilling and borehole image analysis” by Junichi Sugiura) and FIG. 2b (taken from the article “Borehole Imaging Course” by Pavlovic, D. M. and Castillo, H., 2004) illustrate two graphical representations of wells that suffered a spiraling effect, as described above.
FIG. 3 illustrates a second cause of eccentricity of measuring tool 1, which is related to the oval well shape. In this case, even if tool 1 is positioned at the center of the well, the well shape creates varying distances between the tool and the wall since walls 4 of the well will never be equidistant from equipment 1.
FIG. 4 illustrates a final cause of the eccentricity of tool 1, which is related to the eccentric position of tool 1 itself. In this case, even if the well has a perfectly cylindrical shape, the erroneous positioning of the tool 1 generates variable distances between the tool and the wall, since it is closer to one wall 4 than another.
The eccentricity generates an unwanted effect on data acquired due to the change of distance between the transducer and the rock wall along the circumference of the well. Greater or lesser distances between the transducer and the wall result in greater or lesser attenuation of acoustic waves, generating a background of amplitudes that does not reflect the intrinsic characteristics of the rocks but reflects the geometric effect.
Therefore, whatever the cause of such eccentricity, it is known that it undermines the obtaining of acoustic properties intrinsic to the reservoir, so it is important that such a phenomenon is corrected in the amplitude image after the acquisition of the signals that generate this image.
However, with the prior art, there is no known adequate processing of the images generated to correct eccentricity. The prior eccentric correction processing art is known as Eccentering Correction and merely equalizes the amplitudes along lines parallel to the circumference of the image. Although this prior art improves the generated image, it generates sub-horizontal artifacts in regions of the well in which the variance of amplitude is very high (such as fracture zones, vugs and caves). Such artifacts in many cases make it impossible to quantitatively use the corrected image.
Another undesired effect of this correction method results in assigning spurious amplitude values, which generates a loss to the original impedance characteristics of the reservoir.
Thus, it is clear that the prior art lacks a correction method of ultrasonic images that is able to correct any deviations due to the eccentricity of the tool that acquires these images.